Fluororesin tubes are excellent in electric insulation, thermal resistance, releasability, etc., and are thus usable as a surface release layer of a belt used for an image forming apparatus employing an electrophotographic system. Such belt surface release layers are required to be high quality with a thin wall and without variations in wall thickness and diameter.
Heretofore, fluororesin tubes have been produced by melting fluororesin and extruding the molten fluororesin through a die. The extrusion is conducted using either internal- or external-circumference regulating means so that the produced fluororesin tube can maintain its perfect round shape (Patent Literatures 1, 2, etc.).
Recently, there has been a demand for further improved image quality, which requires a sufficiently wide nip width. Therefore, progress towards increasing the diameter of the fixing belt has been made in order to meet this requirement.
With regard to conventional thin-walled fluororesin tubes having a diameter as small as less than ø 100 mm, the extruded tube-shaped resin can retain its stiffness by itself; therefore, a tube with a perfect round shape can be obtained, while achieving sufficient thickness precision. Regarding a tube having a diameter as large as ø 100 mm or more, the thickness of the extruded tube-shaped resin is insufficient for its diameter, resulting in a lack of stiffness causing deflection under the force of gravity. This renders the produced tube incapable of retaining uniform properties and wall thickness, leading to large variations in its properties and thickness in the circumferential direction of the tube. For this reason, producing a thin-walled tube with a large diameter, which can satisfy the market demand, has been difficult.
Additionally, when a fluororesin tube has a high degree of orientation, defects such as cracking or tearing easily occur on its surface during the formation or actual use of a roller or a belt. When a fluororesin tube has a diameter as large as ø 100 mm or more, the degree of orientation varies and tends to increase. Accordingly, improvement therefor needs to be made.
[Citation List]
[Patent Literature]
[PTL 1] Japanese Unexamined Patent Publication No. 6-143389
[PTL 2] Japanese Unexamined Utility Model Publication No. 7-033624